1. Field of the Invention
This invention relates to a cycle-modulating type of dual-tone multifrequency (DTMF) generator, and particularly to a generator for generating a DTMF signal by means of a digital circuit.
2. Description of the Prior Art
A conventional DTMF generator is usually an analog circuit device as shown in FIG. 1, in which the block 1 is the pick-up switch assembly of DTMF voltage signal; the blocks 2 and 3 form into an assembly of tracer and buffer circuits; the block 4 is an analog signal adder; the block 5 is a reference voltage generator. The aforesaid analog circuit is subject to interference or signal distortion because of manufacturing process, temperature or environmental factors; consequently, the output value thereof is seldom conformed to the actual value; further, after an analog circuit being used for a long time, the elements thereof are subject to becoming aged, and the output values thereof will be affected; therefore, the accuracy of a conventional analog circuit for generating DTMF signal is difficult to control, especially during at a lower voltage. Regarding the manufacturing process, the analog circuit has no flexibility in terms of circuit board area because of its characteristics, i.e., the circuit board can be reduced to a given extent only, having no breakthrough further. Moreover, when the telephone line is designed, the column value signal (voltage value) and the row value signal have a ratio gain of 2 dB. But a conventional analog adder can only control the gain in a range from 1-3 dB, i.e., unable to control the gain at 2 dB exactly, and therefore the accuracy thereof is in accurate.